The present invention relates to a knock detection device for an internal combustion engine such as gasoline engine, for a motor vehicle.
When knocking occurs in an engine, a vibration having a characteristic resonance frequency component inherent to the knock is generated. By detecting such vibration with a knock sensor, generation of knocking can be detected.
In a conventional knock detection device, only one specific frequency component having the highest occurrence frequency is extracted from many components contained in the signal obtained from the knock sensor, and the presence of knocking is judged depending upon the magnitude of the extracted signal level.
Knock detection devices relating to the above explained conventional knock detection device are disclosed, for example, in JP-A-59-73750(1984), JP-A-59-125034(1984), and JP-A-60-204969(1985) which corresponds to U.S. Pat. No. 4,612,902, in JP-A-1-178773(1989), and in JP-A-3-47449(1991) which corresponds to U.S. Pat. No. 4,991,553.
The conventional knock detection devices have not necessarily taken into account the existence of a plurality of vibration modes caused in the engine due to knocking and have treated engine vibration due to knocking associated with some of the vibration modes as noises; accordingly, with the conventional knock detection devices, the occurrence of knocking could not be detected accurately, and further, in some engine operating conditions, in particular in a high speed operating condition, for example at more than 4000 rpm, knock detection with the conventional knock detection device was not very effective. Therefore, the capacity of the conventional knock detection device was limited.
In the conventional knock detection device, only a specific vibration frequency component contained in the signal from the knock sensor was extracted and processed with a band pass filter to detect the occurrence of knocking; however, in an actual engine knocking phenomenon, there are a plurality of characteristic resonance vibration frequency components corresponding to a plurality of resonance vibration modes caused in the engine due to knocking.
FIG. 7 schematically illustrates five resonance vibration modes P10, P20, P01, P30 and P11 which typically occur in an engine cylinder for an internal combustion engine due to knocking and characteristic resonance vibration frequencies f10, f20, f01, f30 and f11 correspond to the respective five resonance vibration modes. Since the nodes and loops of pressure variation due to resonance vibration in the engine cylinder appear in the radial and circumferential directions of the engine cylinder, in the symbols representing the respective resonance vibration modes, the nodes of pressure variation are represented by solid lines, including straight lines and circular lines therein, and the loops of pressure variation are represented by + and -. However, as indicated above, in the conventional knock detection devices, only one specific resonance vibration frequency component, such as f10 or f20, which occurs most frequently, is extracted and processed to determine the occurrence of knocking, with the result that the knock detection capacity with the conventional knock detection devices are limited as indicated above.
Further, the characteristic resonance vibration frequencies due to knocking vary dependent upon the nature of the gas in the engine combustion chamber. Namely, although a resonance wavelength in the engine cylinder is determined by the configuration of the engine combustion chamber, which functions as a resonance cavity, the sonic velocity in the combustion gas increases in response to such factors as the combustion temperature and the compression ratio increase, so that the vibration frequencies increase.
Still further, when the compression ratio is increased, this not only causes an increase in the combustion temperature, but also causes variation of the resonance cavity configuration over a long time span, to thereby change the characteristic resonance vibration frequencies of the respective resonance vibration modes due to knocking.
The conventional knock detection devices were provided with a band pass filter as a countermeasure against the above problem in an effort to expand the band of the band pass filter; as a result, the conventional knock detection devices were more likely to be affected by noises which reduced accurate detection of knocking.